Liquid extrusion porosimeter and method

ABSTRACT

A porosimeter evaluates the porosity characteristics (specifically, pore volume, pore distribution and liquid permeability) of a porous sample of material. The porosimeter includes a fluid reservoir located below the sample, and a penetrometer comprising a vessel which catches any fluid displaced from the reservoir of fluid, wherein a level of fluid rises in the penetrometer when additional fluid enters the penetrometer. The sample is preferably wetted, with the same type of fluid which is in the reservoir, prior to placing the sample on the porosimeter. The porosimeter preferably also includes a membrane located between the sample and the reservoir of fluid. The membrane has pores with a size smaller than any of the sample pores. Pore volume of the sample is determined by measuring the change in fluid level in the penetrometer after pressure, which is above the bubble point pressure of the sample but below the bubble point pressure of the membrane, is applied to the sample. Permeability is measured by measuring rate of flow while the liquid level is above the sample.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention pertains to the field of porosimetery, or themeasurement of the porosity of substances. More particularly, theinvention pertains to a liquid extrusion method and apparatus fordetermining the porosity characteristics of a sample.

[0003] 2. Description of Related Art

[0004] The prior art measures the pore volume of a sample using a weightand balance method. Specifically, as shown in FIG. 1, a sample (1) iswetted by water and then placed above a membrane (2). A reservoir offluid (3) is located below the membrane (2). This fluid is the same typeof fluid which is used to wet the sample (1). A vacuum is used to drawthe liquid through the sample.

[0005] In vacuum systems pressure cannot be controlled accurately ormaintained at a constant value. The low pressure causes loss of liquidfrom pores due to evaporation.

[0006] The prior art uses water as the wetting fluid. Water has high airdiffusivity, which increases the air bubbles in a sample, andpotentially yields inaccurate results by increasing the volume of thedisplaced liquid. The viscosity of water is low, which also leads tobubble formation.

[0007] This equation below used for computing pore diameter (D) fromresults of porosimetry shows that accuracy of measurement is determinedby surface tension, γ, and contact angle, θ, of the wetting liquid. Forwater γ is large and changes easily due to contamination and θ is alsolarge and varies appreciably depending upon the nature of the sample.These uncertainties contribute to error.

D=4γ cos θ/P

[0008] The sample (1) has larger pores (4) than the pores (5) of themembrane (2). Vacuum (13) is applied, until liquid is drawn out of thepores (4) in the sample, and into the reservoir of fluid (3). Thedisplaced fluid (7) flows over the top of the reservoir container (8)and is caught in a receptacle (9). The receptacle (9) is on a balance(10), which weighs the amount of the displaced fluid (7). This weightchange is used in combination with calculations known in the art todetermine the volume of the pores (4) in the sample (1). A counterweight(11) on the balance (10) is used to determine the weight change due tothe displaced fluid (7).

SUMMARY OF THE INVENTION

[0009] A porosimeter evaluates the porosity characteristics of a poroussample of material. The sample is preferably wetted, with the same typeof fluid which is in the reservoir, prior to placing the sample on theporosimeter, or the fluid can be poured over the sample in the chamberand pressure applied to force the fluid into the pores of the sample. .

[0010] The porosimeter of the present invention comprises a source ofpressure connected to a pressurizable chamber for holding the sample,and a reservoir of fluid located below the sample, to which is connecteda penetrometer comprising a tube into which fluid displaced from thereservoir of fluid can flow. Thus the level of fluid will rise in thepenetrometer when additional fluid enters the reservoir, and bymeasuring the level of fluid in the penetrometer the volume of fluidentering the reservoir can be measured.

[0011] Te sample is supported by a membrane located between the sampleand the reservoir of fluid. The membrane has a plurality of pores with asize smaller than any of the sample pores, so that the bubble point porediameter of the membrane is smaller than the smallest pore of interestin the sample.

[0012] The pore volume of the wetted sample is determined by applying apressure which is above the bubble point pressure of the sample, butbelow the bubble point pressure of the membrane, and measuring thechange in fluid level in the penetrometer.

[0013] Preferably, a flurocarbon or silicone liquid is used as the fluidin the porosimeter. Flurocarbon and silicone liquids have low surfacetension and the contact angle is zero for many materials. The lowsurface tension enables smaller pores to be measurable. Unchangingsurface tension gives more accurate data. Zero constant contact anglegives more accurate and less uncertain results.

[0014] The same apparatus can be used to measure permeability of thesample by measuring flow versus time when pressure is applied to thesample.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 shows a device for measuring pore volume as known in theprior art.

[0016]FIG. 2 shows a device for measuring pore volume and/orpermeability in an embodiment of the present invention.

[0017]FIG. 3 shows an alternative embodiment of the device of thepresent invention.

[0018]FIG. 4A shows a flowchart of one method of the present invention.

[0019]FIG. 4B shows a flowchart of an alternative method of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0020] An example of a porosimeter of the present invention is shown inFIG. 2. A sample (1) of a material whose porosity characteristics are tobe determined, is located on a membrane (2). Preferably, as shown inFIG. 2, the membrane (2) is supported on a highly porous rigid support(14) to prevent flexing of the membrane due to pressure.

[0021] A liquid is selected that spontaneously fills the pores of thesample and the membrane. The sample and membrane pores (4) are filledwith the fluid when the testing begins.

[0022] The fluid is preferably any fluid which effectively wets thesample (1), and preferably has low air diffusivity. A fluid with low airdiffusivity is preferred because fluids with less air diffusivity do notproduce bubbles which cause inaccuracies in measured volume of displacedfluid.

[0023] Air at pressures below the bubble point pressure of the membranedissolve in the liquid in the pores of the membrane, diffuse through theliquid and form bubbles in the liquid. The air bubbles displace liquidand the volume of displaced liquid increases although there is nodisplacement of liquid from pores of the sample. This error can beconsiderable in case of water in which the air solubility anddiffusivity is high.

[0024] Examples of the fluid to be used include, but are not limited to,fluorocarbon, silicone, or any wetting fluid which the user might use inhis product. Preferably, the fluid has a small (near zero) contactangle, low surface tension, low air solubility, low air diffusion andhigh viscosity.

[0025] Flurocarbon and silicone liquids have low surface tension and thecontact angle is zero for many materials. The low surface tensionenables smaller pores to be measurable. Unchanging surface tension givesmore accurate data. Zero constant contact angle gives more accurate andless uncertain results. Air solubility and diffusion in flurocarbon andsilicone liquids is very low. Flurocarbon and silicone liquids have muchless vapopr pressure than water. Therefore, errors due to loss of liquidfrom pores is minimized.

[0026] The size of the pores (4) in the sample (1) may vary, dependingon the nature of the sample. The membrane (2) needs to be chosen suchthat the smallest pore of interest in the sample is larger than thelargest pore (5) in the membrane (2). Therefore, the membrane (2)preferably has a very small pore size to accommodate many differentsamples (1). An example of a membrane which has been used is Poreticspolycarbonate membrane, catalog No. 13705, from Osmonics, Inc, ofMinnetonka, Minn. Although the pores (5) in the membrane (2) are smallerthan those in the sample (1), the pores (5) in the membrane (2) arepreferably more numerous than the pores (4) in the sample (1), so thatthe permeability of the sample/membrane combination is determined bythat of the sample rather than the membrane.

[0027] The bubble point of a sample (1) is pressure at a point that canovercome the capillary action of the fluid within the pores (4). Thesize of the pores in a material determines the bubble point, or thepressure at which the liquid is extruded or forced out of the pores—thebubble point is inversely proportional to the size of the pores.

[0028] Since the sample (1) has a larger pore size than the membrane(2), the bubble point of the pores (4) in the sample (1) is lower thanthe bubble point of the pores (5) in the membrane (2). Therefore, whensufficient gas or air pressure (6) is applied to exceed the bubble pointof the sample (1), the fluid is forced out of the relatively largerpores (4) in the sample (1), and passes through the relatively smallerpores (5) in the membrane (2). The amount of pressure (6) applied shouldbe high enough to exceed the bubble point of the smallest of the samplepores (4) of interest, but below the bubble point of the membrane (2),so that eventually all of the fluid is forced out of the sample pores(4), but no fluid is forced out of the membrane pores (5).

[0029] A reservoir of fluid (3) is located below the membrane (2). Thefluid in the reservoir (3) is the same type of fluid as the fluid usedto wet the sample (1). The extruded fluid which passed from the pores ofthe sample through the pores of the membrane displaces the fluid in thefluid reservoir (3). Thus, the total amount of fluid displaced from thereservoir will represent the amount of fluid which was trapped in thepores of the sample.

[0030] A penetrometer (25)—a relatively thin tube having a small bore tofacilitate measurement of small volume changes is connected to thereservoir (3).

[0031] In the embodiment of FIG. 2, the penetrometer (25) is made up ofa horizontal portion (26) and a vertical portion (27), which meet atmore or less a right angle. In the embodiment of FIG. 3, a slantedportion (28) replaces the vertical portion, to minimize the effects ofthe weight of the column of fluid (32) on the testing. In eitherembodiment, as fluid is forced through the membrane (2) into thereservoir (3), the level (34) in the penetrometer (25) will rise.

[0032] The change (33) in the penetrometer fluid level (34) may bedetected in a number of different ways. If the tube is made oftransparent material, as noted above, the vertical (27) or slanted (28)portion of the penetrometer can be preferably calibrated by etched orpainted markings, in any convenient scale, which would allow an operatorto directly read the amount of fluid rise. Because the diameter of thepenetrometer tube is known, the volume of fluid in the level rise caneasily be calculated. If desired, the tube can be directly calibrated involume, rather than units of length.

[0033] In a preferred embodiment, the level (34) is read by anelectronic means. As shown in FIG. 2, a magnetic float (30) can beplaced in the vertical tube (27). As the level (34) rises, the magneticfloat (30) position can be sensed by coils or Hall-effect sensors orother means known to the art, and the fluid rise (33) determined.

[0034] In the embodiment using a slanted penetrometer tube (28), asshown in FIG. 3, a float is less practical. In this embodiment, thechange (33) in fluid level (34) can be sensed by a capacitance sensor(31) external to the penetrometer tube (28). The angled portion (29) ispreferably drained periodically if too much fluid enters thepenetrometer (25).

[0035] Before testing, the level (34) of fluid in the penetrometer (25)would be approximately the same as in the reservoir (3). That levelwould be the starting level for the test, if the test is started with afully wetted sample. If the embodiment of the method which wets thesample in the chamber is used to wet the sample, the level in thepenetrometer might change as excess fluid is forced through the sample(1), but at some point when the pressure has forced all of the excessfluid through the sample, but has not yet reached the bubble point ofthe largest pores, the level (34) will stop changing, and that will betaken as the starting level for the test.

[0036] Preferably, the apparatus is maintained at a constant lowtemperature during the testing, which will further limit bubbleformation and lead to more accurate results.

[0037] The apparatus shown can also be used to measure permeability,either as a separate test or subsequent to the measurement of the porevolume. In such an application, the membrane (2) is either absent (ifonly permeability is to be tested) or has a sufficiently high number ofpores (5) such that the permeability of the membrane is higher than thesample (1), and thus does not affect the total permeability of thesample/membrane combination.

[0038] In this embodiment, the apparatus measures permeability in thewetted sample (1) by starting with a quantity of fluid over the sample(2). As the pressure (6) increases, preferably in small steps, the fluidflows through the sample. By measuring the rate of flow through thesample (1) and the applied pressure (6) over time, the permeability ofthe sample (1) can be determined.

[0039] Once the excess fluid has passed through the sample, themeasurements of permeability are complete. If it is desired to measurepore volume in the same run, the liquid level in the penetrometer can bemeasured as a starting point (it is possible that the penetrometer mightneed to be drained or disconnected during or after the flow measurement)and the method of the invention can proceed.

[0040] A flowchart of one method for measuring pore volume using theapparatus described above is shown in FIG. 4A. First, the sample is wetin step (100), preferably by adding a fluid which has low airdiffusivity.

[0041] Once the sample is wet, it is placed on the membrane (2) in step(110). Alternatively, the sample could be placed on the membrane (step(110)) and then wet (step (100)) by putting the fluid on top of thesample (1) and membrane (2).

[0042] As previously mentioned, the pores (5) in the membrane (2) have asmaller pore size than any of the pores (4) in the sample (1).

[0043] The pressure is increased in a controlled manner, preferably insmall steps of a few tenths of a psi, in step (120). Once the pressureexceeds the bubble point pressure, the fluid in the pores (4) begins tobe pushed out of the largest pores (4) in the sample (1).

[0044] The extruded fluid enters the reservoir of fluid (3), displacingfluid already in the reservoir (3). This fluid enters the penetrometer(25) or (25).

[0045] The pressure (6) is continually increased, preferably in smallsteps, until the fluid in the penetrometer (25) or (25) reachesequilibrium. Equilibrium is reached when all of the fluid has beenremoved from of the pores (4) in the sample (1), and the fluid level isno longer increasing.

[0046] Once the fluid in the penetrometer (25) or (25) reachesequilibrium, the fluid level change in the penetrometer is measured instep (130). If penetrometer (25) is used, this step is preferablyaccomplished by sight or by using a magnetic float (30) in thepenetrometer (25). If penetrometer (25) is used, a capacitance meter(31) preferably measures the fluid level change. The pore volume is thencalculated using the fluid level measurement by techniques well known inthe art.

[0047] A flowchart of an alternative method of the invention is shown inFIG. 4B. This method measures the permeability of the sample using theapparatus described above. The apparatus either has no membrane (2) orhas a membrane (2) with a much higher permeability than that of thesample (1). The membrane (2) permeability must not affect thepermeability measurements of the sample (1).

[0048] The sample (1) is filled with fluid in step (140). Permeabilityis measured in step (150). This is accomplished by measuring flow versustime.

[0049] If a user would like to also measure pore volume, steps (120)through (130) from FIG. 4A can be performed to measure pore volume inthe sample (1). This combined method can only be used if a membrane waspresent during the permeability test. Therefore, in step (160) one askswhether or not a membrane was included in the apparatus during steps(140) and (150). If no, the method ends in step (170). If yes, the usermay optionally perform steps (120) and (130) to determine the porevolume of the sample (1).

[0050] Accordingly, it is to be understood that the embodiments of theinvention herein described are merely illustrative of the application ofthe principles of the invention. Reference herein to details of theillustrated embodiments is not intended to limit the scope of theclaims, which themselves recite those features regarded as essential tothe invention.

What is claimed is:
 1. A method of evaluating the porositycharacteristics of a sample of material having a plurality of poresusing a porosimeter comprising a pressurizable sample chamber forholding the sample, a membrane located at a bottom of the sample chamberand having a plurality of pores, wherein the membrane pores have a sizesmaller than any of the sample pores of interest, a reservoir of fluidlocated directly below the membrane, and a penetrometer coupled to thereservoir, wherein a level of fluid rises in the penetrometer whenadditional fluid enters the reservoir, comprising the steps of: a)wetting the sample and membrane with a fluid until the fluid has enteredsubstantially all of the pores in the sample; b) applying a pressure inthe sample chamber which is greater than a bubble point pressure of thesample, but less than a bubble point pressure of the membrane, until thefluid entering the penetrometer reaches an equilibrium; and d) measuringa fluid level change in the penetrometer between a fluid level beforestep (c) and a level after equilibrium.
 2. The method of claim 1,wherein the fluid is a fluid with low air diffusivity.
 3. The method ofclaim 1, wherein the fluid is selected from the group consisting of afluorocarbon or silicone.
 4. The method of claim 1, further comprisingthe step of calculating a pore volume of the sample using the fluidlevel change measured in step (d).
 5. The method of claim 1, wherein theporosity characteristic being evaluated is the pore volume of thesample.
 6. A method of evaluating the porosity characteristics of asample of material having a plurality of pores using a porosimetercomprising a pressurizable sample chamber for holding the sample, amembrane located at a bottom of the sample chamber and having aplurality of pores, wherein the membrane pores have a size smaller thanany of the sample pores of interest, a reservoir of fluid locateddirectly below the membrane, and a penetrometer coupled to thereservoir, wherein a level of fluid rises in the penetrometer whenadditional fluid enters the reservoir, comprising the steps of: a)placing the sample in the sample chamber, on the membrane; b) wettingthe sample and membrane with a fluid until the fluid has enteredsubstantially all of the pores in the sample and membrane; c) adding aquantity of additional fluid above the sample in the sample chamber; andd) measuring flow rate of the fluid and applied pressure over time. 7.The method of claim 6, further comprising the step of calculating apermeability of the sample using the flow rate and pressure versus timemeasurements in step (d).
 8. The method of claim 7, further comprising,after step (d), the steps of: e) applying a pressure in the samplechamber which is greater than a bubble point pressure of the sample, butless than a bubble point pressure of the membrane, until the fluidentering the penetrometer reaches an equilibrium; and f) measuring afluid level change in the penetrometer between a fluid level before step(f) and a level after equilibrium.
 9. The method of claim 8, furthercomprising the step of calculating a pore volume of the sample using thefluid level change measured in step (g).
 10. The method of claim 6,wherein the fluid is a fluid with low air diffusivity.
 11. The method ofclaim 6, wherein the fluid is selected from the group consisting of afluorocarbon or silicone.
 12. The method of claim 6, wherein theporosity characteristic being evaluated is the permeability of thesample.
 13. A porosimeter for evaluating the porosity characteristics ofa sample of material having a plurality of pores comprising: a) apressurizable sample chamber for holding the sample, b) a membranelocated at a bottom of the sample chamber and having a plurality ofpores, wherein the membrane pores have a size smaller than any of thesample pores of interest, c) a reservoir for fluid located directlybelow the membrane; and d) a penetrometer coupled to the reservoir, suchthat if the reservoir is full of liquid, a level of fluid rises in thepenetrometer when additional fluid enters the reservoir through themembrane.
 14. The porosimeter of claim 13, further comprising a fluidhaving low air diffusivity.
 15. The porosimeter of claim 14, wherein thefluid is selected from the group consisting of a fluorocarbon orsilicone.
 16. The porosimeter of claim 13, wherein the porositycharacteristic being evaluated is the pore volume of the sample.
 17. Theporosimeter of claim 13, wherein the membrane has a higher permeabilitythan the sample.
 18. The porosimeter of claim 13, wherein thepenetrometer comprises a tube.
 19. The porosimeter of claim 18, whereinthe comprises a horizontal portion connected to the reservoir and asubstantially vertical portion.
 20. The porosimeter of claim 19, inwhich at least the vertical portion of the penetrometer is transparentand has a plurality of calibrations, such that the fluid level in thepenetrometer can be measured by comparison to the calibrations.
 21. Theporosimeter of claim 19, further comprising a magnetic float in thepenetrometer and a magnetic sensor adjacent to the penetrometer, suchthat the magnetic sensor measures a fluid level change in thepenetrometer.
 22. The porosimeter of claim 18, wherein the tubecomprises a horizontal portion connected to the reservoir and an angledportion, wherein the angled portion meets the horizontal portion at anangle substantially less than ninety degrees.
 23. The porosimeter ofclaim 22, further comprising a capacitance meter adjacent to the angledportion of the penetrometer, wherein the capacitance meter measures afluid level change in the penetrometer.